Close-contact type image sensor and image reading apparatus using the same

ABSTRACT

For preventing occurrence of interference fringes derived from interference of light rays on a contact plane between a contact glass and a cover glass, and thus homogenizing an output wave that may otherwise undergo the adverse effect of interference fringes, a close-contact type image sensor includes a contact glass arranged to come into contact with a read surface of an original, a light source for irradiating light to the read surface, an image forming lens for converging light reflected from the read surface, an image sensor part for reading an image of the original formed on an image plane of the image forming lens, and a cover glass for fixing the light source and the image forming lens at respective predetermined positions in a housing, wherein an air layer is present between the contact glass and the cover glass.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image sensor capable of beingemployed in a facsimile system, copier, scanner, or the like for readinglight reflected from a read surface, and to an image reading apparatususing the image sensor.

2. Description of Related Art

In the past, a close-contact type image sensor using a sensor array toform a read image on a one-to-one reduced scale and thus read an imagefrom a read original with the size of the image unchanged has been usedas an original reading device to be employed in a facsimile system,copier, scanner, or the like.

A known method is, as described in Japanese Patent Publication No. Hei8-34531, such that a rod lens array and a line light source are fixeddirectly to a housing by performing affixation or the like, and then acontact glass is fixed to the housing. However, the work is complex.Besides, since the rod lens array and the line light source are affixedmutually independently, a positional deviation is likely to occur. Thereis the fear of a failure to read an image properly.

FIG. 7 is a sectional view of a known example of a close-contact typeimage sensor in which the above drawbacks are taken into account. Theclose-contact type image sensor has a housing 1 serving as a supportingmeans in which a sensor substrate 10 on which a sensor part 4 composedof a plurality of sensor chips for defining the locations of a pluralityof pixels and for carrying out photoelectric conversion, and aprotective film for protecting the plurality of sensor chips aremounted, and an optical member 3 including light sources 13 attached toan end face thereof for irradiating light to a read original, a rod lensarray 2 that is a lens for forming an image read from the read originalas the pixels on the surface of the sensor part 4, and a contact glass 5serving as an original reading plane, are mounted. Attached to thesensor substrate 10 are an image processing circuit 11 for processing animage signal read by the sensor part 4 composed of the sensor chips anda connection part 12 for externally outputting an output signal of theclose-contact type image sensor. The rod lens array 2 and the opticalmember 3 are pressed to the housing 1 by the contact glass 5. Thecontact glass 5 is fixed to the housing 1 by performing affixation orthe like.

However, in this known example, since the contact glass 5 determines thefocal length of the rod lens array 2, there is no range of choice indetermining the thickness of the contact glass 5. Therefore, a specialglass having a specific thickness must be custom-made.

In recent years, the number of scenes in which color images are handledhas increased with the prevalence of color printers and color displays.There is an increasing demand for reading of color originals rather thanconventional monochrome originals.

When a close-contact type image sensor is used to read a color original,light emitted from a light source and falling on the original readingplane must be multi-color light so that the color original can be read.In general, trichromatic light of red, green, and blue is used as lightincident on the original reading plane. Three light sources mutuallyseparated to be associated with the three colors or a single lightsource formed with a recently-developed white LED for emitting whitelight having the three colors mixed therein is included in theclose-contact type image sensor.

Moreover, studies have been made of a close-contact type image sensorfor reading a color original, in which light reflected from the originalreading plane and segmented into colors is passed through a rod lensarray that is a lens for forming an image as pixels on the surfaces ofsensor chips, guided to the plurality of sensor chips arranged to definethe locations of pixels and designed to carry out photoelectricconversion, and then photoelectrically converted color by color.

An information processing apparatus using the close-contact type imagesensor, for example, a scanner, may adopt either a flat-bed system or asheet-through system.

The flat-bed system is such that a book or any other original is placedon a top glass with an information side thereof facing the top glass,and a close-contact image sensor opposed to the information side ismoved to read image information from the original.

By contrast, the sheet-through system is such that mutually-independentoriginal sheets are moved one by one while brought into close contactwith a close-contact type image sensor.

In the known example, for coping with both the systems, the contactglass 5 must be replaced with another.

The prior art will be described in conjunction with FIGS. 5 and 6. FIG.5 is a sectional view showing the structure of a close-contact typeimage sensor of the sheet-through system. FIG. 6 is a sectional viewshowing the structure of a close-contact type image sensor of theflat-bed system. The structure in accordance with the prior art will bedescribed. That is to say, an optical member 3 serving as a light sourceunit including light sources 13 and a rod lens array 2 are pressed fromabove in the drawing by means of a cover glass 6 and located at desiredpositions in a housing 1. The cover glass and a housing 1 are secured byperforming affixation or the like. As a result, the optical member 3 andthe rod lens array 2 are fixed to the housing 1.

A sensor part 4 mounted on a sensor substrate 10 is formed by placing asensor array on the sensor substrate 10, electrically connected to animage processing circuit 11, resistor, and capacitor, and alsoelectrically connected to a connection part 12 enabling connections withexternal circuits.

In FIG. 5, the contact glass 5 which is arranged to be brought intocontact with an original P is fixed to the cover glass 6. The reasonstherefor will be described below.

(i) The thickness of a glass matching the focal length of the rod lensarray 2 can be set to a selected value more easily by forming the glasswith two glasses of the contact glass 5 and cover glass 6 than byforming the glass with a single glass. Consequently, at least the rodlens array 2 and the optical member 3 can be fixed at the same time.

(ii) For the one kind of housing 1, either the flat-bed system or thesheet-through system can be selected depending on the presence orabsence of the contact glass 5.

In FIG. 6, the contact glass 5 is removed, and a top glass 9 is disposedin such a way as to ensure an optically equivalent structure. Theoptical member 3 serving as a light source unit uses a light guide todiffuse light rays emitted from the light sources (LEDs) 13 located atboth ends in a longitudinal direction of the optical member 3, and toguide the diffused light to an original P. The original P is thenilluminated with the light. Light reflected from the original P passesthrough the rod lens array 2, forms an image on the sensor part 4, andis then converted into an electrical signal. For reading a color image,LEDs for emitting light rays of three colors of red, green, and blue areincluded and lit.

However, as far as the foregoing close-contact type image sensor of theprior art is concerned, even when the surfaces of the contact glass 5and the cover glass 6 shown in FIG. 5 which are in contact with eachother are flattened to as great an extent as mechanically andeconomically possible, a gap of several um deep would exist partly on acontact plane 20 between the contact glass 5 and the cover glass 6.Therefore, interference fringes are produced by interference of light.Accordingly, an output wave undergoes the adverse effect of interferencefringes and exhibits inhomogeneities associated with the interferencefringes.

Moreover, the depth of the gap varies depending on the pressure given bya roller used to transport an original or depending on ambienttemperature or humidity. The output wave varies accordingly. This posesa problem that since correction cannot be achieved properly, a readerror occurs.

For reading especially a color image, when light rays with longwavelengths interfere with one another, interference fringes are likelyto occur. The wavelengths of red, green, and blue light rays becomedifferent because of the interference fringes. Consequently, the problemthat a color deviation occurs in read data becomes outstanding. Theadverse effect of the interference fringes becomes more serious thanthat occurring when a monochrome image is read.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to solve the foregoing problems.

To attain the above object, in accordance with an aspect of the presentinvention, there is provided an image sensor which comprises a contactglass arranged to come into contact with a read surface of an original,a light source for irradiating light to the read surface, an imageforming lens for converging light reflected from the read surface, animage sensor part for reading an image of the original formed on animage plane of the image forming lens, and a cover glass for fixing thelight source and the image forming lens at respective predeterminedpositions in a housing, wherein an air layer is present between thecontact glass and the cover glass. Accordingly, image reading can beachieved with high definition while being unaffected by interferencefringes.

Further, the depth of the air layer is 50 μm or more. Moreover, byremoving the contact glass, the close-contact type image sensor copingwith the sheet-through system can be remodeled to cope with the flat-bedsystem.

Other objects and features of the present invention will be apparentfrom the following detailed description of preferred embodiments thereoftaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a sectional view showing a close-contact type image sensor inaccordance with a first embodiment of the present invention.

FIG. 2 is a sectional view showing a close-contact type image sensor inaccordance with a second embodiment of the present invention.

FIG. 3 is an exploded perspective view showing the structure of theclose-contact type image sensor in accordance with the secondembodiment.

FIG. 4 is a circuit diagram showing a scanner having the close-contacttype image sensor in accordance with the first embodiment.

FIG. 5 is a sectional view showing a close-contact type image sensor ofthe sheet-through system in accordance with a prior art.

FIG. 6 is a sectional view showing a close-contact type image sensor ofthe flat-bed system in accordance with the prior art.

FIG. 7 is a sectional view showing a known example of a close-contacttype image sensor of the sheet-through system.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the drawings.

First Embodiment

FIG. 1 is a sectional view of a close-contact type image sensor inaccordance with the first embodiment of the present invention. Theclose-contact type image sensor has a housing 1 serving as a supportingmeans in which a sensor substrate 10 on which a sensor part 4 composedof a plurality of sensor chips for defining the locations of pixels andcarrying out photoelectric conversion and a protective film, which isnot shown, for protecting the plurality of sensor chips are mounted, anoptical member 3 including light sources 13 such as LEDs for irradiatinglight to a read original, a rod lens array 2 that is a lens for formingan image read from the read original as the pixels on the surface of thesensor part 4, a contact glass 5 serving as an original reading plane,and a cover glass 6 separated from the contact glass 5 by a gap, aremounted. Moreover, attached to the sensor substrate 10 are an imageprocessing circuit 11 for processing an image signal read by the sensorpart 4 composed of the sensor chips, and a connection part 12 forexternally outputting an output signal of the close-contact type imagesensor. Moreover, the cover glass 6 and the rod lens array 2 are incontact with each other.

The focal length of the rod lens array 2 employed in the firstembodiment is 4.8 mm in the air. If one glass that is the contact glass5 is used, the thickness of the contact glass must be 7.2 mm inconsideration of the refractive index of the glass. A glass having sucha thickness is unavailable in standard models and, therefore, becomesvery special and expensive.

However, when the contact glass 5 is combined with the cover glass 6,the thicknesses of the contact glass 5 and the cover glass 6 may be setto 6 mm and 1.2 mm, respectively. Thus, a glass capable of covering thefocal length of the rod lens array 2 can be realized at low cost.

However, when the glasses are merely assembled, the aforesaid problem ofinterference fringes takes place. The reasons therefor will be explainedslightly in detail below. Assume that the depth of an air layer betweenthe contact glass 5 and the cover glass 6 is d, the refractive index ofair is n (approximately equal to “1”), and the wavelength of a lightflux is λ. For example, when light is incident vertically, an effect ofinterference varies depending on whether a quotient given by theexpression below is an integer or half integer.

(nd)/λ  (1)

Accordingly, a transmittance varies. The depth of the air layer, d,varies depending on the flatness of a transparent member, accuracy inmaintaining a gap, or any other factor of differentiating one productfrom another, and is, therefore, different from position to position.The difference leads to a difference in transmittance from position toposition. Eventually, interference fringes appear.

In general, the wavelengths of light emitted from a light source, λ, arenot confined to one wavelength but constitute a spectrum. Assuming thata wavelength half-width of the spectrum is Δλ and a center wavelength ofthe emitted light is λ₀, if the following relationship is established:

[{(nd)/(λ₀−Δλ)}−{(nd)/(λ₀+Δλ)}]>½  (2),

interference in which light rays mutually intensify and interference inwhich light rays mutually attenuate occur among light rays withwavelengths in the spectrum. Consequently, interference fringes becomeindiscernible. The larger the value of the above left side is, the lessdiscernible interference fringes are. Normally, the value should beequal to or larger than “5”.

The following is deduced from the above:

(i) As the depth of the air layer, d, becomes larger, interferencefringes become more indiscernible.

(ii) As the center wavelength, λ₀, becomes larger, interference fringesbecome more discernible.

(iii) As the wavelength half-width, Δλ, becomes smaller, interferencefringes become more discernible.

On the other hand, for reading a monochrome image, a light sourceglowing in green is used generally. However, for reading a color image,light sources glowing in red, green, and blue, respectively, are needed.In this case, interference fringes caused by red light whose centerwavelength λ₀ is long pose a problem.

In the first embodiment, LED chips are used as light sources glowing inred, green, and blue. The center wavelength λ₀ of the LED glowing ingreen with high luminance ranges from about 510 to 530 nm and is thusrather short. An LED for emitting light whose center wavelength israther short or about 600 to 630 nm is used as the LED glowing in red.However, as far as an LED chip capable of glowing with high luminance inthe spectrum is concerned, a wavelength half-width of the spectrum, Δλ,is as small as about 20 nm. This facilitates occurrence of the problemof interference fringes.

In the first embodiment, if a value provided by the above expression:

[{(nd)/(λ₀−Δλ)}−{(nd)/(λ₀+Δλ)}]  (3)

should be equal to or larger than “5”, the depth of the air layer, d,must be equal to or larger than approximately 50 μm.

In the first embodiment, the thickness of the contact glass 5 is 5 mm,the thickness of the cover glass 6 is 1 mm, and the depth of the airlayer, d, is 0.8 mm.

In FIG. 1, while the rod lens array 2 and the optical member 3 are, likethose in the prior art, fixed to the housing 1 by means of the coverglass 6, the contact glass 5 is fixedly secured to the housing 1, and agap 30 is intentionally provided between the contact glass 5 and thecover glass 6, thus preventing occurrence of interference fringes. Thegap 30 should preferably be defined according to the flatness of theparts so that the contact glass 5 and the cover glass 6 will not comeinto contact with each other and will be separated from each other by 50μm or more. Herein, as mentioned above, the gap 30 is 0.8 μm deep. Thisnumerical value is a value permitting the contact glass 5 and the coverglass 6 to be kept apart even when the contact glass 5 is pressed due tothe weight of an original or the rotation of a roller. With the gap 30,a variation of the distance (focal length) from the rod lens array 2 tothe sensor part 4 due to the pressing is quite limited. Therefore, thelevel of a signal representing a read image remains homogeneous.

Second Embodiment

FIG. 2 is a sectional view of a close-contact type image sensor inaccordance with the second embodiment of the present invention. FIG. 3is an exploded perspective view for explaining the structure of theimage sensor.

The second embodiment is an improvement of the first embodimentdescribed with reference to FIG. 1. In view of the fact that a gap ofseveral μm deep might be present between the rod lens array 2 and thecover glass 6 in the first embodiment shown in FIG. 1, the structure ofthe second embodiment is intended to prevent interference fringes fromoccurring due to the gap between the rod lens array 2 and the coverglass 6.

In the second embodiment, in place of the cover glass 6, there isprovided a cover member 7 having a concave part thereof formed as a gap31 in an optical area thereof that is an area in contact with the rodlens array 2. Preferably, the cover member 7 and the rod lens array 2should be separated from each other by 50 μm or more. In the secondembodiment, they are separated from each other approximately by 150 μm.

Likewise, a gap 32 is created between the contact glass 5 and the covermember 7. In the second embodiment, as shown in FIG. 3, the cover member7 is made of a transparent material such as polycarbonate or an acrylic.After the optical member 3 having substrates 8 that include lightsources 13 such as LEDs attached to both ends thereof and also having alight guide member exhibiting high scattering efficiency between thesubstrates 8, and the rod lens array 2 are set to the housing 1, thecover member 7 is snapped down and fitted into the housing 1 from abovein order to fix the optical member 3 and the rod lens array 2 to thehousing 1. Thus, assembling is easy to do. This leads to improvedproductivity.

While the cover member 7 is made of a transparent material, the covermember 7 alternatively may be a sheet metal having a slit, for thepurpose of attaining the same object.

In FIG. 3, the substrates 8 each have an LED mounted as the light source13 thereon, each include a lead over which electric power is supplied,and are fixed to both ends of the optical member 3. After beingassembled, the leads are linked to a substrate 10 of the sensor part 4by performing soldering or the like so that the leads can be linked to apower supply by way of the substrate 10 of the sensor part 4 and theconnection part 12.

Since a transparent material such as polycarbonate or an acrylic is usedas the material of the optical member 3, expansion and contractionderived from an ambient change in heat or humidity are expected. Forminimizing a positional deviation between the light sources 13 and theoptical member 3 caused by expansion and contraction, the substrates 8having the LEDs mounted thereon and the optical member 3 are mutuallyfixed. The fixing method may include caulking, affixation, and snap andfit.

The positions in a longitudinal direction of the housing 1 and theoptical member 3 should preferably be determined in consideration of theforegoing expansion and contraction dependent on temperature andhumidity. The longitudinal center of the optical member 3 is positionedto be fitted with a groove or projection (not shown) formed in thecenter of the housing 1. Moreover, the sensor part 4 is fitted into agroove in the housing 1 and then fixed by performing affixation,caulking, or snap and fit. After the cover member 7 is snapped down andfitted into the housing 1, the contact glass 5 is affixed to the housing1.

The foregoing description has been made by taking the sheet-throughsystem for instance. In the case of the flat-bed system, the contactglass 5 is not included but a top glass is disposed serving as thecontact glass 5.

FIG. 4 is a circuit diagram showing a scanner in which the close-contacttype image sensor in accordance with the first embodiment, for example,is incorporated.

Referring to FIG. 4, a signal processing circuit 41, which includes anA/D converter, is arranged to process a signal outputted from theconnection part 12 of the close-contact type image sensor. An interfacecircuit 42 is arranged to perform bilateral communications with anexternal signal processing apparatus, such as a personal computer, i.e.,to receive commands from the external signal processing apparatus andsend image signals processed by the signal processing circuit 41 to theexternal signal processing apparatus. A controller 43, such as a CPU(central processing unit), is arranged to synchronously control thesignal processing circuit 41, the interface circuit 42 and the lightsources 13.

The structure of the close-contact type image sensor has been describedmainly. The close-contact type image sensor is incorporated in an imagereading part of an image reading apparatus, such as a scanner. Forreading especially a color image, since occurrence of interferencefringes can be prevented, an image signal permitting high image qualitycan be produced.

What is claimed is:
 1. An image sensor comprising: a contact glasssupported by a housing and arranged to come into contact with a readsurface of an original; a light source for irradiating light to the readsurface; an image forming lens for converging light reflected from theread surface; an image sensor part for reading an image of the originalformed on an image plane of said image forming lens; and a cover glassfor fixing said image forming lens at a predetermined position in saidhousing, so that an air layer is formed between the contact glass andsaid cover glass.
 2. An image sensor according to claim 1, wherein adepth of said air layer is 50 μm or more.
 3. An image sensor accordingto claim 1, wherein said image sensor is used in a flat-bed scannersystem or a sheet-through scanner system.
 4. An image reading apparatususing said image sensor according to one of claims 1 to
 3. 5. An imagesensor according to claim 1, wherein a further air layer is formedbetween said image forming lens and said cover glass.
 6. An image sensoraccording to claim 5, wherein a depth of said further air layer is 50 μmor more.
 7. An image sensor according to claim 1, wherein said coverglass further fixes said light source at a predetermined position insaid housing.
 8. An image reading system comprising: a) an image sensorincluding: a contact glass supported by a housing and arranged to comeinto contact with a read surface of an original; a light source forirradiating light to the read surface; an image forming lens forconverging light reflected from the read surface; an image sensor partfor reading an image of the original formed on an image plane of saidimage forming lens; and a cover glass for fixing said image forming lensat a predetermined position in said housing, so that an air layer isformed between said contact glass and said cover glass; b) a signalprocessing circuit for processing an image signal read out from saidimage sensor part; c) an interface circuit for bilateral communicationswith an external signal processing apparatus; and d) a controller forcontrolling said light source, said signal processing circuit and saidinterface circuit.
 9. An image reading system according to claim 8,wherein a further air layer is formed between said image forming lensand said cover glass.
 10. An image reading system according to claim 9,wherein a depth of said further air layer is 50 μm or more.
 11. An imagereading system according to claim 8, wherein said signal processingcircuit includes an A/D converter.
 12. An image reading system accordingto claim 8, wherein said signal processing includes a central processingunit.
 13. An image reading system according to claim 8, wherein saidcover glass further fixes said light source at a predetermined positionin said housing.
 14. An image reading system according to claim 8,wherein a depth of said air layer is 50 μm or more.
 15. An image sensorcomprising: an image forming lens for converging light read from a readsurface of an original; an image sensor part for reading an image of theoriginal formed on an image plane of said image forming lens; and acover glass for fixing said image forming lens at a predeterminedposition in a housing, so that an air layer is formed on alight-incident area of said image forming lens between said imageforming lens and said cover glass.
 16. An image sensor according toclaim 15, further comprising a light source for irradiating light to theread surface of the original.
 17. An image sensor according to claim 15,wherein a depth of said air layer is 50 μm or more.
 18. An image sensoraccording to claim 15, wherein said cover glass further fixes said lightsource at a predetermined position in said housing.
 19. An image sensoraccording to claim 15, wherein said image sensor is used in a flat-bedscanner system or a sheet-through scanner system.
 20. An image readingapparatus using said image sensor according to one of claims 15-19. 21.An image reading system comprising: a) an image sensor including: alight source for irradiating light to a read surface of an original; animage forming lens for converging light from the read surface; an imagesensor part for reading an image of the original formed on an imageplane of said image forming lens; and a cover glass for fixing saidimage forming lens at a predetermined position in a housing, so that anair layer is formed on a light-incident area of said image forming lensbetween said image forming lens and said cover glass; b) a signalprocessing circuit for processing an image signal read out from saidimage sensor part; c) an interface circuit for bilateral communicationswith an external signal processing apparatus; and d) a controller forcontrolling said light source, said signal processing circuit and saidinterface circuit.
 22. An image reading system according to claim 21,wherein a depth of said air layer is 50 μm or more.
 23. An image readingsystem according to claim 21, wherein said cover glass further fixes alight source at a predetermined position in said housing.
 24. An imagereading system according to claim 21, wherein said signal processingcircuit includes an A/D converter.
 25. An image reading system accordingto claim 21, wherein said controller includes a central processing unit.